Polymorphism: Identification with DSC

Each of the modifications of a polymorph has different physical properties. That´s why they have to be characterized before processing, for example using DSC.

How to characterize modifications of a polymorphic substance quickly and easily? Let us demonstrate this on sorbitol, a sugar alcohol found in fruits that is frequently applied as a sweetener in food products. It exists in four anhydrous crystalline phases plus the hydrate. This polymorphism has an influence on the properties of this substance: Each form behaves differently with regard to melting and to water absorption [1].

How to measure:

A sorbitol sample (mass: 3.81 mg) from Sigma-Aldrich was prepared in a Concavus crucible and measured with the DSC 204 F1 Nevio. Three heatings were carried out between -80°C and 150°C at a heating rate of 10 K/min:
  • 1st heating: sample as received;
  • 2nd heating:  after 1st heating and cooling at 10 K/min;
  • 3rd heating: after 2nd heating, cooling at 10 K/min and 24 hours at room temperature.
The DSC measurements were carried out in a dynamic nitrogen atmosphere.

What the curves look like:

Figure 1 displays the DSC curves of sorbitol during the three heating runs.
  • 1st heating: The endothermal peak with an extrapolated onset temperature of 91°C results from the melting of the sample. This temperature is typical for the modification known as a gamma form, which is the most suitable for commercial applications because it is the most stable one.
  • 2nd heating: No melting peak is detected: The sample no longer exhibits any crystalline phase and it is in an amorphous state with the glass transition at -1°C (mid temperature).
  • 3rd heating: One day at room temperature is enough to allow crystallization to take place. However, the peaks detected at 57°C and 81°C (peak temperatures) prove that it is a different crystalline form than the one detected during the first heating. This DSC curve is typical for the modification called crystallized melt. This form is more hygroscopic than the gamma one. However, it is used commercially because of its transparent and glassy appearance, for example, in the production of hard candies.
Figure 1. DSC curves of sorbitol as received (blue), after controlled cooling (pink) and after one day at room temperature (green). The differences in the curves indicate different polymorphic modifications.
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